1. Field of the Invention
This invention generally relates to a method of forming thin film semi-conductors and, more specifically, forming an I-III-VI2 compound semiconductor thin film of chalcopyrite structure.
2. Description of the Prior Art
Semiconductor devices are devices that employ semiconductor materials, which are solid materials that exhibit an electrical conductivity lying between that of a conductor and that of an insulator. Semiconductor devices include, for example, diodes (e.g., light emitting diodes (LEDs)), photovoltaic devices, sensors, solid state lasers, and integrated circuits (e.g., memory modules and microprocessors).
Photovoltaic devices are semiconductor devices that convert photons (e.g. light) into electricity. For example, solar panels include photovoltaic devices that convert sunlight (i.e., photons originating from the sun) into electricity. Due to the ever-increasing demand for renewable energy sources, the market for photovoltaic devices has experienced an average annual growth rate of about twenty five percent (25%) over the previous decade.
Extensive research and development has resulted in photovoltaic materials and devices that are cheaper and more efficient. The cost of power produced by photovoltaic devices has decreased significantly over the past several decades, but must be further reduced to become competitive with alternative power sources, such as coal.
A majority of photovoltaic devices that are commercially available at the present time comprise photodiodes formed in silicon substrates. The performance of such silicon-based photovoltaic devices, is however, inherently limited by physical and chemical properties of silicon. New photovoltaic devices have been created that are based on light-absorbing materials (which may be either organic or inorganic) other than silicon. The number of non-silicon-based photovoltaic devices has steadily increased over the previous two (2) decades and currently accounts for over ten percent (10%) of the solar energy market. Non-silicon photovoltaic devices are expected to eventually replace a large portion of the market for silicon-based photovoltaic devices and to expand the solar energy market itself due to their material properties and efficient power generating ability. In order for solar power to be economically competitive with alternative fossil fuel power sources at their current prices, photovoltaic devices based on photoactive materials other than silicon must be improved and further developed.
Materials other than silicon that can be employed in photovoltaic devices include, for example, germanium (Ge), chalcopyrites (e.g., CuInS.sub.2, CuGaS.sub.2, and CuInSe.sub.2), chalcogenides [Cu(In.sub.xGa.sub.1−x)(Se.sub.yS.sub.1−y).sub.2], cadmium telluride (CdTe), gallium arsenide (GaAs), organic polymers (e.g. polyphenylene vinylene, copper phthalocyanine, fullerenes), and light absorbing dyes (e.g., ruthenium-centered metalorganic dyes). Photovoltaic devices based on such materials have demonstrated greater photon conversion efficiencies than those exhibited by silicon-based devices. Furthermore, some non-silicon photovoltaic devices are capable of capturing a broader range of electromagnetic radiation than silicon-based devices, and as such, may be more efficient in producing electrical power from solar energy than are silicon-based devices.
In particular, compound semiconductor material has attracted much attention most recently because of its application in high efficient photovoltaic devices. For example, chalcopyrite Cu(In,Ga)(S,Se)2 (CIGS) thin film solar cells have reached up to 20.3% power conversion efficiency and show great application potential in photovoltaic devices. Decent conversion efficiency and high chemical stability of CIGS make it a promising p-type material for thin film solar cells. The most common vacuum-based process is to co-evaporate or co-sputter copper, gallium, and indium onto a substrate at room temperature, then anneal the resulting film with a selenide vapor to form the final CIGS structure. An alternative process is to co-evaporate copper, gallium, indium and selenium onto a heated substrate.
These multi-step fabrication techniques involve deposition and then a subsequent annealing in one or more steps in a group VIA or other reactive environment. In order to obtain a viable CIGS photovoltaic device, it is very critical to optimize the metallic precursor layers and choose a suitable selenization temperature profile and duration.
The Se supply and selenization environment is extremely important in determining the properties and quality of the film produced from precursor layers. When Se is supplied in the gas phase (for example as H2Se or elemental Se) at high temperatures the Se will become incorporated into the film by absorption and subsequent diffusion. During this step, called chalcogenization, complex interactions occur to form a chalcogenide. These interactions include formation of Cu—In—Ga intermetallic alloys, formation of intermediate metal-selenide binary compounds, and phase separation of various stoichiometric CIGS compounds. Because of the variety and complexity of the reactions taking place, the properties of the CIGS film are difficult to control. When a selenium element is used in its solid powder form close to the metallic precursor layer, problems of heterogeneity of this element may arise. The big selenium molecule clusters impinge on the growth surface in the conventional evaporation based method. The thermal energy and chemical activity of these larger molecules are relatively low, which leads to the poor crystal quality of CIGS films.
Differences exist between films formed using different Se sources. Using H2Se yields the fastest Se incorporation into the absorber; 50 at % Se can be achieved in CIGS films at temperatures as low as 400 C. By comparison, elemental Se only achieves full incorporation with reaction temperatures of 500° C. and above. Below 500° C. films formed from elemental Se were not only Se deficient, but also had multiple phases including metal selenides and various alloys. Use of H2Se also provides the best compositional uniformity and the largest grain sizes. However, H2Se is highly toxic and is classified as hazardous to the environment. It implies severe constrains on the mass production processes.
Moreover, IIIB element in depth of the initially deposited material is subsequently much different through the depth of the final semiconductor absorption layer after the selenization as much of IIIB element is pushed to bottom of the layer in the final semiconductor material.
Stephane et al., (U.S. Published Pat. Appl. No: US 2009-0130796 A1) describe a method for sulfurization and selenization of electrodeposited CIGS films by thermal annealing.
Serdar et al., (U.S. Pat. No. 8,153,469) disclose a method for forming CIGS solar cell absorber layer on continuous flexible substrates based on a two-stage process (metallic precursor layer deposition and S/Se high temperature annealing process).
Basol (U.S. Published Pat. Appl. No: US 2009-0130796 A1) discloses a method of and an apparatus for converting metallic precursor layers into CIGS solar cell absorbers on a surface of a flexible roll.
Rakesh et al., (U.S. Published Pat. Appl. No: US 2012-0122268 A1) describe a method of fabricating a CIGS nanocrystal precursor layer and selenizing the nanocrystal precursor layer in a selenium containing atmosphere.
Lakshmikumar and Rastogi (J. Appl. Phys. 76, 3068 (1994)) describe a plasma assisted two stage method of preparing selenide semiconductor thin film using elemental selenium vapor and vacuum evaporation technique.
Tao et al., (Chin. Phys. Lett. 27, 028101 (2010)) demonstrate a plasma-assisted selenization method of preparing CuInSe2 films under different working pressures.
Ishizhuka et al., (U.S. Pat. No. 8,012,546) disclose a method of preparing semiconductor thin films by cracking selenium with plasma to generate radical selenium, and using the radical selenium in the process of forming the semiconductor film.
However, selenization processes involving selenium solid powders have suffered from the heterogeneity of Se element along the CIGS thin films. Although the use of H2Se has provided good compositional uniformity, H2Se is a highly toxic gas that requires severe constraints on the mass production processes. Also, all plasma assisted selenization methods are based on vacuum evaporation techniques that are also very limited in mass production processes.